Scenes from the Lamar Valley Bison Rut 2019 in Yellowstone

The rut is an exciting time of year. Bellows fill the air, the group buzzes with activity, and male-male clashes can be seen. As the fall leaves begin to turn yellow and orange, the mating activities being to die down. But only 285 days later, reddish-orange baby calves are born, bringing excitement once again to the landscape.

Breeding is strongly seasonal and very promiscuous. Bison are not monogamous, but rather polygynous, meaning bulls are will mate with more than one female, but females will only mate with one bull.

Beginning in early June as the breeding activities start, mature bulls will join mixed-sex groups to compete for mating opportunities. This is when the mood changes and the deep bellows of bull bison can be heard across the landscape. Bulls begin to court females through a variety of behaviors such as sniffing female genital areas and face-to-face lip curls. Wallowing behaviors also increase, where males will roll violently on the ground to display aggression. This wallowing behavior can cause so much dust to rise that the herds can disappear behind clouds of dust! A bull's tail also indicates mating status and behavior. A tail held high in a "question mark" fashion indicates a threat or challenge. This question mark signal is commonly seen as bulls lead a female away for mating.

Once a bull finds a receptive female, he will form a tending bond to keep other bulls ways from her. These bonds can last from a few minutes to a few days, depending on when the female will accept copulation. During these tending bonds, the bull demonstrates intolerance for all other group members through a variety of bellowing, wallowing, and threat displays. Unlike other species, such as elk, elephant seals, and baboons, that form harems---animal group consisting of one male and multiple females---male bison will remain part of the large group during the rut expect for these temporary tending bonds. It is documented that higher male copulation rates are associated with higher social dominance. Females are more willing to copulate with larger, more dominant males than with smaller males.